The present invention relates to a method for mounting electronic and/or mechanical components on a printed board, a guide element and an electronic or mechanical component.
For the purpose of this patent application, the term xe2x80x98electronic componentxe2x80x99 is used for any electric or electronic components, such as capacitors, resistors, transistors and integrated circuit chips that could be mounted to a printed board to create a printed board assembly. Examples of component included in the term xe2x80x98mechanical componentsxe2x80x99 are shields, housings and supports for the shields.
The assembly process for an apparatus that comprises a printed board assembly involves several automatic assembly steps and often also at least some manual assembly steps. One of the assembly steps is to mount standard electronic components, and sometimes mechanical components, on a printed board. This is often done by applying a soldering paste on the printed board and automatically place the electronic and mechanical components on the printed board in a so-called SMD-line by a so-called pick and place machine. After that, the components are fixed to the printed board by soldering, for example in a soldering furnace, and then the printed board assembly is fastened to an element, e.g. a chassis or a base plate. However heavier mechanical or electronic components, such as three-way splitter-combiners, large shields and circulators/isolators, are often mounted manually. Hereinafter in this part of the description, these heavier mechanical or electronic components are called xe2x80x98heavy componentsxe2x80x99. This is done after the ordinary lighter components have been surface-mounted and soldered to the printed board. After the heavy components have been placed on the printed board, the whole assembly is transported on a conveyor belt to a screw driving cell where an automatic screw machine fastens the heavy components to the printed board and the element. The fastening with screws is necessary in order to ensure that the heavy components stay in place on the printed board so that an electric connection between the printed board and the heavy component is guaranteed also when the assembly is subjected to large vibrations. The screw machine fastens screws at predetermined positions corresponding to predetermined positions of holes in the heavy components, the printed board and the element. The positions of the heavy components, printed board and base plate are now very important since the automatic screw machine typically demands a tolerance zone of xc2x10.1 mm for the positions of the holes in the printed board, the element and the heavy components, in a horizontal plane in relation to an outer frame of reference as well as in relation to each other, to be able to centre the screw in the hole and engage in corresponding threads in the holes. If the position of a hole deviates more than 0.1 mm, the screw may not engage in a receiving thread and it may also damage the heavy components, the printed board and/or the element as it is forced downwards by the screw machine.
To ensure a sufficiently accurate positioning of a heavy component on the printed board, separate guide pins are pressed down in another set of holes in the element. These guide pins also correspond to through-holes in the printed board and when the heavy component is guided towards the printed board, the guide pins guide the heavy component with the help of a set of corresponding guide holes in the heavy component. The insertion of a guide pin in one of the corresponding holes in the element is often done manually and the guide pin is secured to the hole by a press fit, which demands very small limits of tolerance for the contacting surfaces of the guide pin and the wall of the hole in the element. Moreover, the element, provided that it is e.g. a chassis, is often surface covered with a coating material in order to improve the electric conductibility between the element and the printed board assembly. This coating material often creates bulges around the edges between the holes for the guide pins and the surface, as well as bulges inside the holes. In order to allow proper press fit, these bulges need to be smoothed. Therefore, small limits of tolerance for the guide pins and the corresponding holes in the element generate higher fabrication costs than guide pins and holes with larger limits of tolerance. Also, the manual insertion makes great demands upon a worker when inserting a guide pin, since the press fit easily gives a slight inclination from the desired guide direction of the guide pin. The inclination might cause inaccurate positioning of the printed board assembly, which makes it even more difficult to get a very important accurate positioning of the heavy component in relation to the printed board, since predetermined pads have to get in contact with corresponding legs or tabs on the heavy component. Of course the same demands are true for an automatic guide pin inserting machine, if it is to replace the worker.
Alternatively, the element is provided with integrated guides for the guide holes in the printed board. Thereby the step of inserting a guide pin in a corresponding hole is avoided, but causes other disadvantages. The element, to which the printed board assembly is to be fastened, is often made of metal in order to conduct heat away from the printed board assembly in use and to improve the earthing. Such metal elements, such as chassis, are fabricated in advanced pre-programmed automatic machines, such as a CNC-machine, which is expensive to use and maintain. Small details, such as protruding integrated guides on a flat surface, give a much longer, in many cases unacceptable, fabrication time in the CNC-machine. Also, larger pieces of raw material have to be used.
U.S. Pat. No. 5,978,229-A discloses an apparatus and a process for mounting integrated circuit packages or housings for a integrated circuit on circuit boards. The packages or housings, which consists of several parts, have a frame with guide pins on its bottom surface that mate with holes in the circuit board. Although this solution may be another alternative when mounting heavy components, it suffers from similar disadvantages as the latest alternative above, i.e. the fabricating time and cost for a frame with guide pins are unacceptable to many manufacturers. Furthermore, these integrated guide pins are easily bent and make it more difficult to pack, store and transport the frames. Also, U.S. Pat. No. 5,978,229-A gives no suggestions on how to fix the circuit packages or housings on the circuit board, and the arrangements used in the document are not sufficient for heavy components.
It is therefore a general object of the present invention to provide a method, a guide element and an electronic or mechanical component for mounting and fastening the electronic or mechanical component on a printed board or a printed board assembly in order to solve the above mentioned accuracy problems and disadvantages.
Another general object is to provide an improved method, guide element and an electronic or mechanical component that make the mounting and fastening of a heavy component more effective in terms of cost and assembly time.
The invention therefore provides a method for mounting at least one electronic or mechanical component on a printed board. The method comprises the steps of: surface-mounting at least one guide element on the printed board or placing the at least one guide element in a dead end hole in the printed board, fastening the at least one guide element to the printed board by soldering before guiding the at least one electronic or mechanical component towards the printed board, and placing the at least one electronic or mechanical component on the printed board with the help of the at least one guide element. Thus the difficult manual or automatic insertion of separate guide pins in an element, such as a base plate, is avoided. Also, there is no need for guide pins fixed to a frame, housing or the like for mounting the mechanical or electric component on the printed board, which saves fabrication time and costs.
Suitably, the method comprises the steps of placing the printed board on an element, such as a base plate or a chassis, and fastening the at least one electronic or mechanical component to the printed board by at least one fastener, for example at least one screw that is screwed into to each other corresponding holes in the element, the printed board and the at least one electronic or mechanical component. Hereby is achieved that a electric or mechanical component are fastened to the printed board in a way that reduces the risk of displacement between the printed board and the electric or mechanical component when the whole assembly is subjected to vibrations.
Preferably, the method comprises the steps of mounting at least one second electronic or mechanical component on the printed board and fastening the at least one second electronic or mechanical component to the printed board by soldering before guiding the at least one electronic or mechanical component towards the printed board. This means that smaller or xe2x80x98standardxe2x80x99 components that do not need to be mounted to the printed board with the help of guide pins suitably are placed during the same step as the guide elements and also are soldered to the printed board during a mutual step. Advantageously, the at least one guide element and the at least one second electronic or mechanical component are mounted by an automatic apparatus, such as a so-called pick and place machine.
The invention also provides a guide element for the method described above. The guide element comprises at least one solder surface adapted to be soldered to a printed board and at least one guide surface adapted to guide at least one electronic or mechanical component into a position in relation to the printed board, to which the at least one electronic or mechanical component is to be fastened. Preferably the at least one solder surface is flat and extend substantially perpendicular to a longitudinal axis of the guide element in order to give an even soldered joint. Also, the solder surface may be positioned at a first end section.
To facilitate insertion of the guide element in a corresponding hole in the at least one electronic or mechanical component, the guide element comprises a second end section with a substantially conical or frusto-conical shape. If required, the second end section comprises at least one slot, which divides the end section into substantially axially extending fingers that are deflectable substantially radially inwards.
Suitably, the guide element comprises at least one flange that positions the guide element in a desired direction relative to the printed board when the guide element is placed on the printed board. The at least one flange extends substantially perpendicular to a longitudinal axis of the guide element and comprises the at least one solder surface. Furthermore, the guide element may have a cylindrical guide surface and as an alternative, the solder surface may be ring-shaped.
Moreover, the invention also relates to an electronic or mechanical component to be fastened to a printed board according to the method described above. The electronic or mechanical component comprises at least one first through-hole with a substantially circular cross section adapted to receive a guide element that is soldered to the printed board, at least one second through-hole with an oblong cross section adapted to receive another guide element that is soldered to the printed board, and at least one third through-hole adapted to receive a fastener for fastening the component to the printed board. This facilitates the positioning of the at least one electronic or mechanical component, since the at least one electronic or mechanical component only have to be precisely positioned in relation to only one guide element.
Preferably the electronic or mechanical component is a circulator/isolator, splitter-combiner unit, or a shield for electromagnetic fields.